Wireless communication is the transfer of information between two or more points that are not connected by an electrical conductor. Most common wireless communication technologies, such as radio, use electromagnetic waves typically in the radio frequency (RF) ranges. Information, such as sound, video, or other data, is transmitted by systematically modulating at least some property of a RF carrier. Modulation techniques are well known and in general include modulating the amplitude, frequency, or phase of the RF carrier.
Wireless communication is generally achieved through the following process. A source generates a message that when represented in an electrical waveform is referred to as a baseband signal. A transmitter modulates the baseband signal for efficient transmission, which also typically includes amplification. The modulated amplified signal is fed to a transmit antenna. The transmit antenna converts the modulated signal from a signal propagating along a transmission line into a signal propagating through free space. At the destination, a receive antenna converts the signal propagating through free space into a signal propagating on a transmission line. The transmission line feeds a receiver that typically amplifies and demodulates the received signal to recover the baseband signal.
The prominence of wireless data communications technologies, such as Wi-Fi®, WiMAX® and cellular data services, including 3G (W-CDMA, EDGE, CDMA2000®, etc.), and 4G LTE, continues to increase as consumers continue to adopt and rely on mobile computing platforms, such as laptop, smart phone and tablet computing devices. (Wi-Fi is a registered trademark of the Wi-Fi Alliance Corporation of 3925 W. Barker Lane, Austin, Tex. 78759. WiMAX is a registered trademark of the WiMAX Forum Corporation of 12264 El Camino Real, Suite 402, San Diego, Calif. 92130. CDMA2000 is a registered trademark of Telecommunications Industry Association of 2500 Wilson Boulevard, Suite 300, Arlington, Va. 22201).
Antennas are necessary components in wireless communication systems. Generally, antennas are reciprocal devices that are classified by their electrical characteristics. As a reciprocal device, an antenna's electrical characteristics are the same for both transmission and reception. Example antenna characteristics include resonant frequency, bandwidth, gain, radiation pattern, and polarization. Such antenna characteristics are important to antenna designers and play an important part in the operation of a wireless communication system. The vast majority of antennas are based on the half wavelength dipole antenna.
An antenna radiates efficiently at its resonant frequency. Typically, an antenna element also radiates at odd multiples of a quarter wavelength of its fundamental resonant frequency, although not as efficiently as at its fundamental resonant frequency. In general, antenna efficiency and antenna size are directly related.
An antenna has a purely resistive impedance at its fundamental frequency, but most antenna applications require using an antenna over a range of frequencies. The bandwidth of an antenna specifies the range of frequencies over which its performance does not suffer due to poor impedance match. In general, antenna bandwidth and antenna size are directly related.
Gain is a parameter which measures the degree of directivity of an antenna's radiation pattern. More specifically, the antenna gain (sometimes referred to as power gain) is defined as the ratio of the intensity (power per unit surface) radiated by the antenna in the direction of its maximum output, divided by the intensity radiated at the same distance by a hypothetical isotropic antenna. The gain of an antenna is a passive phenomenon; power is not added by the antenna, but simply redistributed to provide more radiated power in a certain direction than would be transmitted by an isotropic antenna.
The radiation pattern of an antenna is a plot of the relative field strength, or gain of the electromagnetic wave emitted by the antenna at different angles. It is typically represented by a three dimensional graph, or polar plots in the three principal planes (e.g., XY, YZ and XZ) or simply in horizontal and vertical planes.
An isotropic radiator is typically defined as “a hypothetical lossless antenna having equal radiation in all directions.” The isotropic antenna (or isotropic radiator) is an ideal theoretical antenna. It is considered to be a point in space with no dimensions and no mass. Although the isotropic radiator is a theoretical antenna and does not physically exist, it is often used as a reference for expressing the directive properties of actual antennas. The pattern of an ideal isotropic antenna, which radiates equally in all directions, would look like a sphere.
A directional antenna is one “having the property of radiating or receiving electromagnetic waves more effectively in some directions than in others.” The term “directional” is usually applied to an antenna whose maximum directivity is significantly greater than that of a half-wave dipole.
Many antennas, such as monopoles and dipoles, have a directional radiation pattern that is non-directional in one plane (typically the azimuth plane) and directional in an orthogonal plane (typically the elevation plane). Since such antennas emit equal power in all directions in one plane (e.g., the azimuth or horizontal plane), the plot of the radiation pattern in that plane (azimuth) approximates a circle. However, such antennas are directional in the orthogonal plane (e.g., the elevation plane or vertical plane). As a result, a three dimensional plot of the radiation pattern of such an antenna is a torus or donut-shaped. This type of pattern—an essentially nondirectional pattern in one plane (typically, azimuth) and directional pattern in any orthogonal plane (typically, elevation)—is typically designated as “omnidirectional.” An omnidirectional pattern is then a special type of directional pattern.
Omnidirectional antennas are only weakly directional antennas and are typically used when the relative position of the other communication station is unknown, arbitrary, or changes often. Also, omnidirectional antennas are typically used at lower frequency where a directional antenna would be too large and too expensive or where a directional antenna is simply not required. One example of an omnidirectional antenna is the vertical antenna or whip antenna, often but not always a quarter wavelength long. A dipole antenna is similar but consists of two quarter wavelength conductors extending in opposite directions, with a total length that is roughly a half a wavelength long at the resonant frequency of the device.
Directional or beam antennas, which radiate more power in a particular direction, are used when additional gain or the directionality of the antenna's beam being known is useful. Directional antennas include parabolic reflectors, horn radiators and Yagi-Uda antennas.
The polarization of an antenna is the orientation of the electric field (E-field) of the electromagnetic wave with respect to the Earth's surface and is determined by the physical structure and orientation of the antenna. For example, a dipole antenna has a linear polarization; when mounted vertically it has a vertical polarization, and when mounted horizontally it has a horizontal polarization. The reflection of electromagnetic waves off of terrestrial objects generally affects polarization.
Polarization is the sum of the orientation of the E-field vector over a period of time projected onto an imaginary plane perpendicular to the direction of travel of the electromagnetic wave. Polarization is in general elliptical, meaning that the polarization of the electromagnetic wave varies in an elliptical direction over time. There are two special cases of elliptical polarization, namely linear polarization—where the ellipse collapses into a line—and circular polarization—where the two axes of the ellipse are equal. Linear polarization is usually created by an antenna forcing the electric field of the emitted electromagnetic wave in a particular orientation. Usually linear polarizations are either vertical polarization or horizontal polarization.
It is important that polarized antennas be matched so that the maximum amount of power can be transferred between the transmitter and receiver. Therefore, vertically polarized transmit antennas should be matched with vertically polarized receiving antennas and horizontally polarized transmit antenna should be matched with horizontally polarized receiving antennas.